This invention relates to the polymerization of 2-pyrrolidone and more particularly to the formation of polymers which are white in color and have thermal characteristics such that they may be melt extruded when SO.sub.2 is used as the chain initiator.
The formation of polymers of 2-pyrrolidone involving the use of alkaline catalysts under substantially anhydrous conditions was first disclosed in U.S. Pat. No. 2,638,463. The best catalysts are the alkali metal salts of 2-pyrrolidone which are best formed from the alkali metal hydroxides. The lithium salt is very sparingly soluble and therefore is not a good catalyst and since both rubidium and cesium hydroxides are very expensive, in actual practice either the sodium or potassium salt is normally used.
Subsequent patents, for example U.S. Pat. No. 2,809,958, further disclose the need for a co-catalyst or "activator" for the polymerization. U.S. Pat. No. 3,174,951 discloses the use of SO.sub.2 as an activator and U.S. Pat. No. 3,721,652 discloses the use of CO.sub.2. Both of these activators result in the formation of a polypyrrolidone having superior thermal characteristics not found in polymers formed when other activators are used. While the polymer formed when CO.sub.2 is used is white, that formed when SO.sub.2 is used is ordinarily yellow unless carefully controlled amounts of SO.sub.2 are used under certain critical temperatures as disclosed in U.S. Pat. No. 4,105,645. The use of SO.sub.2 is advantageous however in that lower temperatures for the polymerization may be used as well as much smaller amounts of the activator. It is therefore very desirable to provide a method of preventing the formation of yellow polymer since the color greatly limits its use.